A CNC program instructs a milling cutter rotatably driven by a machine tool to cut a workpiece along a cutting path. Without adaptive control of at least the milling cutter's feed rate relative to a workpiece, the machine tool is required to develop an ever increasing torque for the milling cutter to cut successive workpieces in accordance with the CNC program due to its progressively worsening tool wear.
Automatic torque responsive monitoring of the tool status of milling cutters is known in the art, for example, U.S. Pat. No. 4,208,718 to Chung and U.S. Pat. No. 4,802,095 to Jeppesson.
U.S. Pat. No. 4,208,718 to Chung describes an automatic monitoring method in which a tool status indication of a milling cutter is determined as a function of the length of cutting time under increased target horsepower as a percentage of the total cutting time for a machining operation.
U.S. Pat. No. 4,802,095 to Jeppesson describes an automatic monitoring method in which a resultant side loading force FRES, having tangential FT and radial FR components. during a milling operation is measured directly along with its FT component and, on the basis of which, its FR component is determined mathematically for providing an indication of tool wear.
In U.S. Pat. No. 4,547,847 there is described an adaptive control for machine tools which provides tool monitoring functions under a subroutine TLfiNTR illustrated in FIG. 13H and described in col. 35, line 64 to col. 38, line 49. The subroutine TLMNTR assumes a basically linear dulling process and requires several cutting parameters from the operating NC program, such as feed rate and spindle speed. In addition, the subroutine requires either a known or constant depth of cut which, in the case of the former, is compensated for in a linear fashion. As such, the subroutine is suitable for predicting tool life in relatively straightforward turning applications.